The following is related generally to the optical components used in optical communication networks, and specifically to optical devices that can amplify optical signals.
Erbium-Doped Fiber Amplifiers (EDFAs) or Praseodymium-doped fiber Amplifiers (PDFAs) are widely deployed in optical networks, in the 1550 nm or 1310 nm wavelength windows, respectively. FIG. 1 illustrates the multiple optical components that are typically included in a prior art EDFA or PDFA. The optical power from the pump laser light source 102 is combined with the input signal 101, by a wavelength-division multiplexing (WDM) coupler 104. The combined input signal and pump laser light then passes through a section of fiber 103 that has been doped with erbium or praseodymium ions in its core. The pump laser light excites the erbium or praseodymium ions embedded in the erbium-doped (or praseodymium-doped) fiber 103 to a higher energy level. The optical input signal 101 then induces stimulated emission and is therefore amplified to create the output signal. However, amplified spontaneous emission (ASE) noise is also generated simultaneously, and creates noise on top of the amplified input signal 101. Thus the output signal 106 consists of an amplified input signal, as well as the ASE noise component. An isolator 105 is located after the erbium-doped or praseodymium-doped fiber 103. This isolator 105 is intended to prevent the back scattering power out of the downstream optical fiber and other components from re-entering the EDFA or PDFA. This unwanted back scattering power would otherwise be amplified, and would therefore interfere with the EDFA's (or PDFA's) normal characteristics and performance. Also shown in FIG. 1 is a pump laser monitoring port 107.
FIG. 2A illustrates the principle of operation of an Erbium-Doped Fiber Amplifier (EDFA). The figure shows three energy levels, labeled E0, E1, and E2, of Er3+ ions in silica glass. Each of the energy levels are split into multiple levels or bands, via the Stark splitting process, as described in “Erbium-Doped Fiber Amplifiers, Fundamentals and Technology”, Chapters 8 and 9, P. C. Becker, N. A. Olsson, and J. R. Simpson, Academic Press, 1999, for example. The difference between any two of the energy levels is labeled with the wavelength (or wavelength range) of the photons that correspond to that energy level transition. The upward arrows indicate the wavelengths at which the EDFA can be pumped, in order to excite the erbium ions to the indicated higher energy level. For example, a 1480 nm pump laser can be used to excite the erbium ions from the E0 level to the E1 level, whose life time is very long, on the order of 10 msec, such that population inversion between the E0 level and the E1 level is suitable for stimulated emission. The downward arrow from E1 to E0 represents the wavelength range of photons emitted due to spontaneous and stimulated emission, amplifying the input signal. Because the E1 and E0 energy levels are split into bands, a range of wavelengths can be amplified, shown in FIG. 2 as 1520-1560 nm.
Similarly, a 980 nm pump laser can be used to excite the erbium ions from the E0 level to the E2 level. The ions that have been raised to the E2 level quickly transition to the E1 level, via a non-radiative spontaneous emission process. The transition of these ions from the E1 level to the E0 level results in amplification of input signals in the 1520-1560 nm range, via stimulated emission. For a variety of reasons, pumping at 980 nm is more efficient than pumping at 1480 nm.
Pump sources of wavelengths lower than 980 nm can also be used, including visible light. FIG. 2B shows a more complete view of the energy levels of Er3+ ions. The wavelength scale corresponds to the wavelength being emitted when Er3+ ions transit from a given energy level to the ground state. The energy level diagram illustrates that Er3+ ions can absorb light from approximately 1500 nm, down to less than 400 nm. Lower-wavelength pump sources (i.e., lower than 980 nm) excite the erbium ions to higher energy levels than are shown in FIG. 2A, but the overall process for amplifying the input signal via stimulated emission is similar. Also, for input signals of different wavelength ranges, different rare-earth elements, or other materials, including various metals, may be used, with correspondingly different pump source requirements. For example, to amplify input signals in the 1310 nm wavelength range, praseodymium ions have energy level transitions that are in the appropriate wavelength range. Amplifying an input signal of visible light is also physically feasible, as long as the pumping wavelength is shorter than that of the input light, and suitable dopants are used. Broadband sensitizers can also be used, as an aid to the energizing of the dopant material. (More detail on broadband sensitizers can be found, for example, in “Broadband Sensitizers For Erbium-Doped Planar Optical Amplifiers: Review”, A. Polman and F. van Veggel, Journal of the Optical Society of America B, Vol. 21, Iss. 5, May 2004.).
As has been occurring with cell phones, more and more components are being squeezed into individual optical modules, with the same limited volume, in order to save space, and also to upgrade the performance of network control centers. Fiber splicing between separate fiber optic components is cumbersome, and also occupies space. It is therefore highly desirable to integrate multiple optical components into a single package. Prior art EDFAs and PDFAs, as illustrated in FIG. 1, typically make use of separate WDM coupler and isolator components, and also incorporate a length of doped fiber (typically 10 to 20 meters in length) with bend radius limitations. Further, the pump laser source is either an external component, or else it must be integrated into the EDFA housing. These considerations limit the size (and cost) reductions that can be achieved with typical prior art optical amplifiers that are based on the use of doped fibers. Semiconductor optical amplifiers (SOAs) can provide size advantages, but typically have performance limitations, due to higher levels of amplified spontaneous emission (ASE) noise, as well as a variety of non-linear behaviors. It is therefore desirable for the multiple component elements of the doped fiber (or more generally, doped waveguide) optical amplifier to be physically integrated into as few components or elements as possible, but without the performance limitations of semiconductor optical amplifiers.